1. Field of the Invention
This invention relates to a method of arranging the charges of explosive in a multi-charge blast for breaking up or displacing a geological mass, and of timing successive detonations of the charges, by which the ground vibration produced by the blast at one or more chosen outlying locations is reduced, and by which the explosion energy available for increased fragmentation or displacement of the burden can thereby be increased.
2. Description of the Related Art
In the conventional practice of blasting in a geological formation, a pattern of boreholes is drilled into the formation, explosives and detonators are loaded into the boreholes, the resulting charges are then confined with stemming of aggregate, and the explosive charges are then detonated to break up and displace a portion of the nearby rock or soil. This blasting action is the result of high transient gas pressures applied to the borehole walls by the gaseous products of detonation. These pressures produce compression waves that propagate outward from the boreholes and are partially absorbed by the processes of fragmentation and displacement. But waves that escape from the vicinity of the pattern of boreholes in the form of ground vibration can damage outlying structures, interfere with outlying operations or annoy outlying residents. Furthermore, such ground vibration carries energy that was expensive to produce but was not used to do useful work,
The use of time delays between initiations of the charges is an established method of increasing desired fragmentation and displacement, of decreasing undesired damage to a remaining rock face, and of decreasing ground vibration at locations distant from the blast. The charges used in conventional blasting have a typical length of 10 meters and a typical detonation velocity of 5000 m/sec, so that a typical charge is consumed by its detonation in only 2 milliseconds. Delay times used between charges, in order to allow time for sufficient movement of the rock between detonations, are typically an order of magnitude longer, Therefore the ground vibration typically produced at locations distant from such blasts is characterized by a long wavetrain of substantial amplitude which contains vibrational energy produced by the successive, short, well-separated in time, bursts of pressure exerted against the rock by the short, well-separated explosions. Some success has been obtained in reducing ground vibration and improving the blasting action from such blasts by calculating the power or amplitude spectrum from a digital recording of the vibration produced at an outlying location by a single small charge shot in the vicinity of the planned blast and then choosing time delays between charges that are equal to periods of vibration from the small charge that contributed relatively little energy to the spectrum and are not close to the natural periods of vibration of vulnerable structures near the outlying location. Time delays chosen in this way have no consistent or recognized relationship to the length or detonation velocity of a charge, to the velocity of propagation of the ground vibration, or to the difference in range from particular locations in the charges to the outlying location where ground vibration is to be reduced. Also, the time delays between charge initiations chosen in this way are typically much longer than the time required for detonation to consume a charge. Thousands of blasting patterns and time delay arrangements have been proposed in the past, of which those disclosed in U.S. Pat. Nos. 3,295,445, 3,903,799, and 4,770,097 may be cited. But in spite of the limited success obtained through the use of present practices, at many blasting operations a pressing need remains for further reductions in ground vibration or for making larger blasts with no appreciable increase in ground vibration, or for increasing the efficiency with which the energy of the explosive is used to do useful work in the form of increased fragmentation and/or throw.
Some of the prior art teaches the use of extremely high effective detonation velocities, achieved through the use of multiple points of initiation in the same charge, to obtain improved blasting action and/or reduced ground vibration. For example, U.S. Pat. No. 3,457,859 teaches that the use of multiple points of initiation in each charge results in improved fragmentation, and U.S. Pat. No. 4,382,410 teaches that the use of multiple points of simultaneous initiation in a charge surrounded by an air annulus results in both increased rock fragmentation and reduced ground vibration. In contrast, U.S. Pat. Nos. 5,071,496 and 5,099,763 teach the use of an explosive of unusually low detonation velocity together with a conventional arrangement of time delays to obtain increased throw of the burden and reduced ground vibration. Other examples of the use of explosives of very low detonation velocity are given in U.S. Pat. No. 4,864,933 which teaches the use of such explosives for stemming, in place of aggregate, and U.S. Pat. No. 4,864,933 which teaches their use for stimulating wells, but like the rest of the prior art these patents do not recognize any relationship between the velocity of detonation and the optimum time delay between detonations for reducing ground vibration. Blasting with black powder, which also has a very low velocity of propagation, is old in the art and it is still used for blasting dimensioned stone. But its use in general rock blasting became obsolete long before the development of blasting with accurate, short time delays. No disclosure is known of its use with accurate delays chosen to take advantage of its low velocity in order to reduce ground vibration.